Arc rotation rate sensor and interlock



Oct. 21, 1969 STONE ARC ROTATION RATE SENSOR AND INTERLOCK Filed Dec. 29, 1966 .FEEIOw mww mh wv 53;: Si Cizow 102%.?552 hozwmzo r INVENTOR ALBERT RIVINGTON STONE A ORNEY United States Patent 3,474,289 ARC ROTATION RATE SENSOR AND INTERLOCK Albert Rivington Stone, Laurel, Md., assignor to the United States of America as represented by the Secretary of the Navy Filed Dec. 29, 1966, Ser. No. 605,943 Int. Cl. H051) 37/02, 39/04, 41/36 U.S. Cl. 315151 8 Claims ABSTRACT OF THE DISCLOSURE The present invention generally relates to a control system and apparatus for monitoring the rate of motion of an are traveling between electrode means and for extinguishing the arc should the rate of motion decrease below a predetermined value. In one embodiment of the present invention, a plasma arc generator, having a pair of spaced, annular electrodes which are adapted to generate a rotating arc, is provided with a photocell sensor elfective to produce an output pulse each time the rotating arc passes a predetermined point in its travel around the electrode pair. Suitable control circuitry which is responsive to the time between pulse outputs from the photocell sensor is effective, on a fail-safe basis, to extinguish the arc if its rotation rate decreases below a predetermined value. The control circuitry is adapted to be very quick acting if the arc stops completely.

The present invention generally relates to a control system and apparatus for monitoring the rate of motion of an arc traveling between electrode means and for extinguishing the arc should the rate of motion decrease below a predetermined value.

Various devices, such as plasma arc generators, for example, employ an are which rotates between a pair of spaced, annular or ring-like electrodes. It often occurs, during operation, that the arc rotation rate decreases below a desired value or that the arc stops completely and, unless suitable means are provided for extinguishing the arc, the electrodes will be destroyed by are erosion.

With the above in mind, it is proposed in accordance with the present invention to provide a system which senses the rotation rate of a rotating arc and is effective, should the rotation rate decrease below a predetermined desired value, to interrupt the electrode energizing circuit and thus extinguish the arc. More specifically, in accordance with the illustrated embodiment of the present invention, to be described in more detail hereinafter, a photocell sensor is positioned in the wall of a plasma arc generator such that it receives light from the are once during each rotation of the arc between a pair of spaced, annular electrodes. Consequently, the photocell produces a series of output electrical pulse signals at a pulse repetition rate directly proportional to the rotation rate of the arc.

These electrical outut pulses from the photocell are subsequently applied to a capacitor timing circuit such that the timing capacitor is discharged each time an output pulse occurs. In this manner, the capacitor is controlled to charge to a voltage which is directly proportional to the interpulse spacing and inversely proportional to the arc rotation rate. This capacitor voltage is monitored and subsequently used to control a silicon controlled switch circuit which, in turn, controls the energization of an interlock relay whose contacts are connected in the supply circuit to the electrodes. As soon as the capacitor voltage indicates that the rotation rate of the arc has decreased below a predetermined (and adjustable) value, the silicon controlled switch is fired when the next photocell output pulse occurs and the relay is energized to open-circuit the electrode supply and extinguish the are.

If, on the other hand, the photocell completely ceases to produce output pulses, either because the arc has stopped rotating or because of photocell failure, the timing capacitor will charge to its maximum voltage value. When this occurs, the silicon controlled switch is immediately fired (rather than waiting for the next photocell pulse) to energize the interlock relay and rapidly extinguish the arc.

One object of the present invention is to provide a system responsive to the rate of travel of a traveling are capable of extinguishing the arc when the rate of travel decreases below a predetermined value.

Another object of the present invention is to provide a control system comprising a photocell sensor means responsive to the light emitted by a rotatable arc, eifective to extinguish the arc when its rotation rate decreases below a predetermined value.

Another object of the present invention is to provide a control system of the type described particularly adapted for use in conjunction with a plasma arc generator wherein the plasma arc is produced to rotate around and between a pair of spaced, annular electrodes; said control system being effective to extinguish the are if its rotation rate decreases below a predetermined, adjustable value.

A further object of the present invention is to provide a control system of the type described which is fail-safe in operation; i.e., it is effective to rapidly extinguish the are if the arc stops rotating or the sensor means (photocell) becomes inoperative.

Other objects, purposes and characteristic features of the present invention will in part be pointed out as the description of the invention progresses and in part be obvious from the accompanying drawing which illustrates partly in block diagram form and partly in schematic form, one embodiment of the control system of the present invention particularly adapted for use with a so-called plasma arc generator, a portion of which is also illustrated.

Referring now to the drawing, the plasma arc generator generally comprises a cylindrical chamber, a wall portion of which is illustrated at 10, and a pair of spaced, ring-like electrodes 11 and 12 which support a rotating arc 13 therebetween. By way of example, one form of plasma arc generator with which the proposed control system of the present invention might be used is described in the US. patent to Hunt et 211., No. 3,274,424, issued Sept. 20, I966. The electrodes 11 and 12 extend through the wall 10 of the arc generator for energization from a suitable electrical power supply (not shown).

In accordance with the present invention, a suitable photocell 14a is mounted in a suitable housing 14 which extends through the wall 10 of the plasma arc generator in such a position that the photocells field of view is re stricted to one corner of the electrode structure 11-12, as represented by the dotted line. In other words, the photocell 14a receives light from the rotating are 13 only once during each rotation of the are 13 and, in response, produces a corresponding output pulse across its load potentiometer 15. Thus, as the are 13 rotates around the electrodes 11-12, the photocell 14a produces a series of output pulses whose pulse repetition rate is indicative of the rotation rate of the are 13.

The potentiometer arm 16 picks off any desired amplitude of output electrical pulses from the photocell 14a and applies them to a suitable A.C. amplifier 17. Subsequently, the amplifier output pulses from the amplifier 17 are sup-plied as input to a conventional Schmitt trigger circuit 18 which shapes the input pulses into narrow, square pulses of constant amplitude, for application to a one-shot multivibrator circuit 19. The one-shot 19 is triggered in response to these input square pulses from the Schmitt trigger 18 and produces a constant width, positive output pulse for each input pulse.

The output pulses from the one-shot 19 are coupled, through resistor 20, to the base of an npn. transistor stage 21 that is connected in multiple across a timing capacitor 22. Consequently, whenever an output pulse is produced by the one-shot 19 and the transistor 21 is caused to conduct, the capacitor 22 is discharged to ground through collector resistor 23.

This capacitor 22 is normally connected, during the time interval between output pulses from the one-shot 19, in a direct charging circuit which extends from to ground and includes resistor 24. In this manner, the capacitor 22 is permitted to charge to a peak voltage whose value is directly proportional to the time between output pulses from the one-shot 19; i.e., the interpulse spacing, and therefore inversely proportional to the rotation rate of the generator arc 13.

The voltage across the capacitor 22 (essentially a series of ramp pulses) is monitored by a pair of field effect transistors 25 and 26, each having a very large input impedance so as not to appreciably load the capacitor '22. More specifically, the capacitor voltage is applied to the gate electrode G of field effect transistor 25, through resistor 27, and produces a corresponding output voltage signal across the parallel resistance combination 28-29 which connects the source electrode S of transistor 25 to ground via Zener diode 30. The Zener diode 30 furnishes the desired biasing voltage to the field effect transistor 25.

Load resistance 29 is actually a potentiometer having an adjustable arm 31 effective to pick-off any desired portion of the output ramp voltage signals from the field effect transistor 25 and couple them, through capacitor 32, as input to a suitable A.C. amplifying stage 33. The output from the amplifier 33 is subsequently applied to a Schmitt trigger circuit 34 whose output is connected, through capacitor 35 and resistor 36, to a tunnel diode 37 and silicon controlled switch 38 combination which results in an input sensitivity response that is essentially temperature insensitive.

The pick-off arm 31 is adjusted so that as soon as the rotation rate of the are 13 decreases below a predetermined desired value, the voltage at the input to the Schmitt trigger 34 will rise sutficiently so that the next input ramp signal will pulse the trigger 34 and fire the silicon controlled switch 38. Once the silicon controlled switch 38 has fired, it causes energization of the serially connected control relay 39 and thereby interrupts the electrode supply circuit 40-41 (assumed to be connected in series between the electrodes 11-12 and their power supply), to extinguish the are 13. Moreover, once the silicon controlled switch 38 has fired, it preferably holds the relay 39 picked up until a suitable reset switch means (not shown) is operated. A lamp 42 is connected in multiple with the control relay 39 to provide suitable visual indication as to when the relay 39 is operated.

As mentioned previously, the system of the present invention is also fail-safe; i.e., the are 13 is rapidly extinguished if the photocell 14a fails to function properly or if the are 13 stops rotating. More specifically, the field effect transistor 26 also monitors, through resistor 43, the voltage across the timing capacitor 22. Consequently, as soon as the are 13 stops rotating or the photocell 14a fails, the voltage across the capacitor 22 will reach a maximum D.C. level. When this occurs a corresponding output voltage is produced across the output resistor 44 of the field effect transistor '26 and is coupled, through diode 45, resistor 46 and Zener diode 47, to the tunnel diode-silicon control switch combination 37-38. This voltage is effective to immediately fire the silicon controlled switch 38 and energize the control relay 39, to the extinguish the are 13. Thus, this portion of the proposed system is somewhat faster operating than the portion, including Schmitt trigger 34, which waits for and depends upon a subsequent input signal from the photocell 14a to pulse the trigger 34 and fire the silicon controlled switch 38.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In combination with a traveling arc device including a pair of spaced, ring-like electrodes between which a rotating arc occurs in response to energization of said electrodes from an electrical power source,

means for sensing the presence of said are at a predetermined point during each rotation of said arc around said electrodes,

said sensing means being effective to generate a discrete output signal pulse each time said are is at said predetermined point,

control means operably connected between said electrodes and said electrical power source and being predisposed to denergize said electrodes and extinguish said arc in the absence of an output signal pulse from said sensing means, and

means responsive to the output of said sensing means for preventing the operation of said control means only so long as said sensing means continues to generate said output signal pulses at a rate greater than a predetermined value.

2. The combination specified in claim 1 wherein said sensing means is a photocell positioned relative to said electrodes such that it receives light from said are only once during each rotation of said arc, whereby said photocell generates a series of output electrical pulses indicative of the rotation rate of said arc.

3. The combination specified in claim 1 wherein, said predisposed control means comprises,

a capacitor,

a charging circuit connected to continually attempt to charge said capacitor towards a predetermined voltage level at a predetermined time rate, and

first circuit means responsive to the voltage charged on said capacitor for denergizing said electrodes and extinguishing said are if said capacitor voltage reaches said predetermined level, and

wherein said preventing means comprises a discharge circuit means connected across said capacitor and rendered effective to discharge said capacitor each time an output signal pulse is generated by said sen- Slng means,

said predetermined level of capacitor voltage being attained by said capacitor when the rotation rate of said are is slower than desired.

4. The combination specified in claim 3 wherein said discharge circuit means comprises,

a transistor having base, collector and emitter electrodes,

said capacitor being connected between the collector and emitter electrodes of said transistor, and

circuit means connected to said sensing means for applying the output signal pulses generated by said sensing means to the base of said transistor.

5. The combination specified in claim 3 wherein said first circuit means comprises,

first and second field effect transistors each having a gate electrode connected to receive said capacitor voltage, a drain electrode connected to a supply voltage, and a source electrode connected through a resistance means to ground and at which an output voltage is produced corresponding to said capacitor voltage,

a trigger circuit means connected to receive the output voltage from said first field effect transistor and rendered effective to produce an output trigger pulse by the generation of an output signal pulse from said sensing means following the obtainment by said capacitor of said predetermined voltage level,

a switch circuit means operably connected to be selectively actuated by either the output trigger pulse from said trigger circuit or the output voltage from said second field eflect transistor, and

a relay rendered effective when said switch circuit means is actuated to deenergize said electrodes.

6. The combination specified in claim 5 wherein said switch circuit means comprises a silicon controlled switch.

7. The combination specified in claim 5 wherein" said resistance means connecting the source electrode of said first field eifect transistor to ground is a potentiometer permitting any desired portion of the output voltage produced by said first field eiiect transistor to be connected to said trigger circuit.

8. The combination specified in claim 5 wherein the output voltage from said second field etfect transistor effective to actuate said switch means occurs when said capacitor is permitted to charge up to a maximum voltage level in the absence of any output pulses from said sensing means and whereby said electrodes are rapidly deenergized if said sensing means fails or said arc has stopped rotating.

References Cited UNITED STATES PATENTS 1,893,504 1/1933 Nicolson 315--151X 

